In recent years, for a welding process for resistance welding a work (an object to be welded) by using a resistance welding machine, various methods and apparatuses have been proposed to determine the welding quality of the work (i.e., evaluate the quality of a welded portion of the work) (for example, Patent Literatures 1 and 2).
Patent Literature 1 discloses the following method. During a current change period in which an instantaneous value of welding current changes, the welding current and an instantaneous value of inter-tip voltage are first detected. Then, an instantaneous value of apparent inter-tip dynamic resistance is calculated based on both the detected values. A change rate of a dynamic resistance instantaneous value of the inter-tip dynamic resistance at an arbitrary time during the current change period is calculated. Thus, the welding quality of a resistance welded portion is evaluated by use of change characteristics of the dynamic resistance instantaneous value change rate calculated in correspondence with the number of generations during the current change period.
Patent Literature 2 discloses the following method. A moving average value of voltage applied to a welding electrode of an alternating resistance welding machine at predetermined time intervals is first calculated as a moving average voltage value. Then, a moving average value of current flowing in the welding electrode of the alternating resistance welding machine at predetermined time intervals is calculated as a moving average current value. A moving average resistance value is subsequently calculated based on the moving average voltage value and the moving average current value. Thus, the calculated moving average resistance value is compared with a previously set threshold to determine whether or not splash occurs in a joined portion by the resistance welding.
In Patent Literatures 1 and 2, as mentioned above, the welding quality is determined based on the resistance value between welding electrodes (tips). However, during welding of the work, there is a case where contact resistance between the work and the welding electrode varies. For instance, in the case where the work is pressed by a movable gun arm provided at its distal end with a welding electrode to resistance-weld the work, the gun arm may vibrate in a pressing direction during resistance welding. This vibration of the gun arm may cause variation in contact resistance between the work and the welding electrode. In this case, the variation in contact resistance is reflected in a resistance value between the welding electrodes (tips).
However, the variation in contact resistance between the work and the welding electrode is completely unrelated to the change of a welding state (welding quality) of a welded portion (a joined portion). Accordingly, the conventional technique of determining whether the welding quality is good or bad based on the resistance value between the welding electrodes (tips) in which the contact resistance variation is reflected could not appropriately determine the welding quality of the work.